1. Field of the Invention
The present invention relates to a cooling system for a water-cooled internal combustion engine. More particularly, the present invention relates to a cooling system having a pressure-regulating valve and a coolant return passage for controlling pressure of coolant in the cooling system, and to an internal combustion engine and a motorcycle incorporating the described cooling system.
2. Description of the Background Art
There are several known cooling devices (cooling systems) for a water-cooled internal combustion engines. Such cooling devices include a radiator cap detachably provided for replenishing coolant to a coolant system, a pressure-regulating valve including a high-pressure valve and a low-pressure valve provided with the radiator cap for adjusting a pressure of coolant in the cooling system, and a reservoir tank fluidly connected with the radiator cap.
An example of such cooling device for a water-cooled internal combustion engine is disclosed in the Japanese Patent Document JP-A-2007-2678.
According to the cooling device for the water-cooled internal combustion engine, as disclosed in the Japanese Patent Document JP-A-2007-2678, when cooling-water pressure inside the cooling system becomes equal to or greater than a predetermined value, the high-pressure valve of the radiator cap is released and coolant from the cooling system is discharged into the reservoir tank. Hence, cooling-water pressure inside the cooling system is lowered so as to prevent the coolant pressure from being elevated to a predetermined value or more.
Further, when a temperature of coolant in the cooling system is lowered and the cooling-water pressure inside of the cooling system is lowered to a predetermined value or below the predetermined value, the lower pressure valve of the radiator cap is released. Hence, coolant inside the reservoir tank flows in the cooling system so as to possibly prevent the cooling-water pressure inside the cooling system from being lowered to the predetermined value or below the predetermined pressure.
With respect to the cooling device of the water-cooled internal combustion engine according to the Japanese Patent Document JP-A-2007-2678, when a vehicle is stopped for a long time in an idling state after performing a normal operation, the cooling ability of the radiator is largely lowered due to the absence of traveling wind.
Hence, due to absence of traveling wind, temperature of the coolant is elevated, and the coolant pressure inside the cooling system is also elevated. When pressure of coolant is elevated to a value greater than or equal to a predetermined value, the high-pressure valve of the radiator cap is released, and coolant is discharged from the radiator to the reservoir tank.
When the motorcycle is operated to travel thereafter, the radiator is sufficiently cooled by the traveling wind such that the temperature of coolant is lowered. When pressure of coolant inside the cooling system is lowered to a value less than or equal to a predetermined value, the low-pressure valve of the radiator cap is released, and coolant returns to the cooling device from the reservoir tank.
However, in the system as disclosed in the Japanese Patent Document JP-A-2007-2678, the radiator cap is arranged upstream of the radiator. Accordingly, even when a quantity of coolant inside the cooling system is decreased, coolant in the cooling system is not sufficiently replenished since coolant which flows upstream of the radiator is pressurized by the water pump.
Accordingly, the pressure of coolant which flows in the vicinity of the radiator cap is higher than the pressure of coolant disposed over (circulated through) the whole cooling system. Hence, it is difficult for coolant to return to the cooling device when the motorcycle is in a traveling state.
The present invention has been made to overcome such drawbacks as discussed above. Accordingly, it is one of the objects of the present invention to provide a cooling system for a water-cooled internal combustion engine which can rapidly return coolant to the engine as needed, even when a motorcycle is in a traveling state, thus enhancing the cooling performance of the cooling system.